In general, the loading device such as a front loader and the like installed in the working vehicles comprises a pair of left and right lift arms, a bucket attached to heads of the lift arms, cylinders for lifting the lift arms, and cylinders for rotating the bucket. The front loader is attached to brackets installed in the both sides of a bonnet of a tractor that is one of the working vehicles. The cylinders for lifting the lift arms are installed at a lower side of the lift arm of the front loader, and a link for the bucket is installed at an upper side of the lift arm. The link is connected to the cylinder for swinging the bucket through an intermediate link attached to the above-mentioned bracket and to the upper portion of the lift arm. The telescopic motion of the cylinder swings the bucket.
Conventionally, with respect to structures of such a front loader, Japanese Patent Laying-Open No. 2005-16022 discloses that a box-shaped lift arm is used for lifting the loader in order to secure its own strength.
However, in the conventional box-shaped lift arm, four plates are needed to form the box-shape, and the length of welds for joining the plates are long, and this causes an increase in cost. When the lift arm has a plate-like structure, it is necessary to increase the plate thickness in order to take a large load, which increases the weight of the working machine and increases costs in correspondence to the increase of weight.